javascript - Typescript:检查变量的真实性不会对未定义进行类型防护吗？

Question

class Base {}

function log(arg: number) {
    console.log(arg);
}

function fn<T extends typeof Base>(
    instance: Partial<InstanceType<T>>,
    key: keyof InstanceType<T>,
) {
    const val = instance[key];
    if (val) {
        log(val);
    }
}

TS Playground :https://www.typescriptlang.org/play?#code/MYGwhgzhAEBCkFNoG8C+AodAzArgO2ABcBLAez2hFIHMAKMAJ2oC5o8cBbAIwQYEoU6aMOjByEUiAQA6KnUbU+AbnQZs+ImQpY8AHgAq0BAA9CCPABMYhAJ4AHBKSxxEAPlpCRxPBEJgCCKwACowkYCC6AJI+fgH69ggGrq4ANJ7CANYINqxZNk7Q0b7+wAjxDklpAsjpouKE0ABu4dAAvNDexQEA2nkAuioiHc60zSDVtUNyo+HKtRgYQA

我得到:

Argument of type 'InstanceType<T>[keyof InstanceType<T>] | undefined' is not assignable to parameter of type 'number'.
  Type 'undefined' is not assignable to type 'number'.

if (val)不应该防范undefined吗？如果我将其更改为 log(val ?? 0)，它就会起作用。

另外，我很惊讶 log(val ?? 0) 有效。 val 可能是一个非数字的真值，但 log() 需要一个数字。为什么这不会引发错误？

Answer 1

问题是well known 。您可以从此comment了解类型检查器内部到底发生了什么。 :

The core problem is that the narrowed type of fooAnchor is not really speakable. Its unnarrowed type is Partial<FooAnchorMap>[keyof TMap], which isn't a union that we can remove undefined and null from to produce some other type that would be provably-assignable to HTMLElement.

有work in progress来解决它。但到目前为止，您只剩下解决方法了。

function fn<T extends typeof Base>(
    instance: Partial<InstanceType<T>>,
    key: keyof InstanceType<T>,
) {
    const val: InstanceType<T>[keyof InstanceType<T>] | undefined = instance[key];
    if (val) {
        log(val);
    }
}

playground link

您将得到正确的错误: Type 'InstanceType<T>[string]' is not assignable to type 'number'.